The present invention relates to a filter circuit and, more particularly, to a higher-order filter circuit such as a second-order filter circuit used for small signal processing.
Second-order filter circuits used for small signal processing include various circuit systems such as "TUNABLE WIDEBAND ACTIVE FILTER" disclosed in U.S. Pat. No. 5,225,790.
FIG. 40 shows an active filter disclosed in FIG. 2A of U.S. Pat. No. 5,225,790. This active filter comprises a plurality of transistors, two capacitors, and a plurality of resistor elements. This circuit has three transfer functions under predetermined conditions. The first transfer function represents a BPF (Band-Pass Filter) characteristic, the second transfer function represents a through-pass filter characteristic, and the third transfer function represents an LPF (Low-Pass Filter) characteristic having a zero point. In this active filter, selectivity Q for these characteristics and a cutoff frequency fc can be independently set.
However, as shown in FIG. 40, this active filter requires a resistor network having resistors 215, 230, and 232 serving as both input resistors and positive feedback resistors, and many circuit elements, resulting in high power consumption. This active filter can attain only the three filter characteristics. To realize another filter characteristic such as an HPF (High-Pass Filter) characteristic or an LPF characteristic not having any zero point, this active filter must be combined with another arithmetic circuit. This increases the number of circuit elements used and the power consumption. Further, in this active filter, Q cannot be externally controlled upon integration.